1、同步异常的产生和返回的指令 : svc/hyc/smc/eret
这里重点强调一下ERET，以arm64为例，调用该指令后，PSTATE恢复SPSR_ELn的值，PC恢复ELR_ELn的值.
代码示例：
在ATF中，exit_el3函数中，当一切寄存器准备好之后，调用ERET指令，恢复PC指针。跳转回Normal-EL1或secure-EL1
func restore_gp_registers_callee_eret
ldp x4, x5, [sp, #CTX_GPREGS_OFFSET + CTX_GPREG_X4]
ldp x6, x7, [sp, #CTX_GPREGS_OFFSET + CTX_GPREG_X6]
ldp x8, x9, [sp, #CTX_GPREGS_OFFSET + CTX_GPREG_X8]
ldp x10, x11, [sp, #CTX_GPREGS_OFFSET + CTX_GPREG_X10]
ldp x12, x13, [sp, #CTX_GPREGS_OFFSET + CTX_GPREG_X12]
ldp x14, x15, [sp, #CTX_GPREGS_OFFSET + CTX_GPREG_X14]
ldp x18, x19, [sp, #CTX_GPREGS_OFFSET + CTX_GPREG_X18]
ldp x20, x21, [sp, #CTX_GPREGS_OFFSET + CTX_GPREG_X20]
ldp x22, x23, [sp, #CTX_GPREGS_OFFSET + CTX_GPREG_X22]
ldp x24, x25, [sp, #CTX_GPREGS_OFFSET + CTX_GPREG_X24]
ldp x26, x27, [sp, #CTX_GPREGS_OFFSET + CTX_GPREG_X26]
ldp x28, x29, [sp, #CTX_GPREGS_OFFSET + CTX_GPREG_X28]
ldp x30, x17, [sp, #CTX_GPREGS_OFFSET + CTX_GPREG_LR]
msr sp_el0, x17
ldp x16, x17, [sp, #CTX_GPREGS_OFFSET + CTX_GPREG_X16]
eret
endfunc restore_gp_registers_callee_eret
2、armv8新增的ELR寄存器
- AArch64 : Provides a 64-bit program counter (PC), stack pointers
(SPs), and exception link registers (ELRs) - AArch32 : Provides 13 32-bit general-purpose registers, and a 32-bit
PC, SP, and link register (LR). The LR is used as both an ELR and a
procedure link register
也就是说,再aarch64中，ELR做为exception link registers，在异常返回时恢复PC的指针；在aarch32中，使用LR替代ELR
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